1. Field of the Invention
The invention relates to a dispersion of a nanoparticulate mixed oxide of SiO2 with at least one further metal oxide in a matrix monomer, to a dental composite with the dispersion as a precursor and obtainable by curing the dispersion, and to the use of the dispersion of the invention as a precursor for dental composites. The invention further relates to a method for preparing such a dispersion.
2. Description of Related Art
Dental composites are composite materials comprising a polymeric organic phase, in particular a resin matrix, and fillers materials, with preferably inorganic particles being used as a filler material. A dispersion of filler materials in a monomer is used as a precursor for such a dental composite. The precursor is provided in liquid or paste form, i.e. the filler particles are incorporated into a liquid organic phase. The liquid organic phase includes monomers which may be transformed into the resin matrix by polymerization. The corresponding dental composite is formed by curing the dispersion, i.e. by polymerization of the monomers. The physical properties of the dental composite are primarily determined by the type and proportion of inorganic filler materials and the shape, size, and size distribution thereof. By using a suitable filler material, adverse effects such as polymerization shrinkage of the matrix or water absorption thereof may be compensated for or mitigated.
Also, a suitable filler material may reduce the thermal expansion coefficient of the composite. At the same time, compressive strength, tensile strength, bending strength and abrasion resistance as well as the elastic modulus of the composite may be adjusted. These effects correspond with the content of filler material in the composite.
When using the dispersion as a precursor for dental composites, e.g. for a dental filling, polymerization of the monomers is usually induced by light in the visible or UV range. In terms of mechanical properties, in particular longevity of the fillings, a highest possible degree of polymerization throughout the entire volume of the filling is desired. Therefore it will be advantageous, in particular for polymerization of deep fillings, i.e. fillings with a comparatively large layer thickness, if the composite material exhibits high transmittance in the visible range. Scattering loss which may be caused by scattering at rather large filler particles, for example, or by different refractive indices of the filler material and the matrix, will adversely affect translucency.
At the same time, the composite material is desired to be as opaque as possible for X-rays, i.e. the material should have a high X-ray opacity to permit diagnosis using X-rays.
The first commercially available dental composites contained ground glass powders as a filler material. Average grain size was between 50 and 100 μm. However, the size of these macrofillers caused high abrasion. Glass chips broken out from the filling caused craters in the surface of the filling, and, moreover, the broken out glass chips acted as emery and thus increased abrasion of the filling.
By combining fumed silica as a nanofiller and ground glass particles as a microfiller, so-called hybrid fillers can be obtained which allow for a higher solids content in the composite. This makes it possible to obtain composites that exhibit good polishability, high abrasion resistance, and good mechanical strength.
Conventional melting and grinding processes allow to obtain glass powders with particle sizes of less than 1 μm. A drawback of this preparation method, however, is that in addition to components providing for the functional properties, in particular refractive index and X-ray opacity of the filling body, the glass composition also comprises components which ensure meltability in a technically relevant range and which suppress crystallization. However, this will generally involve an increase in the refractive index of the material, thereby restricting, in terms of type and proportion, the choice of components relevant for X-ray opacity, which for their part also cause an increase in refractive index of the material. Furthermore, the preparation of glass powders or glass particles in nanoscale form, i.e. having a particle diameter of less than 100 nm, is generally not economically efficient, due to the grinding times required. Moreover, in particular with long grinding times, there is always a risk that the materials leach out during the grinding process and thus lose their desired properties.
Patent document EP 1 711 433 B1 (WO2005/075348), by contrast, describes dental composites comprising a mixed oxide of SiO2 obtainable by flame spray pyrolysis, and at least one further oxide of any of the elements of a group including Y, La, Ta, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, as an X-ray opaque component. However, the corresponding dental composites only exhibit a comparatively low X-ray opacity and poor transparency for light in the visible range.
Published patent application DE 10 2006 045 628 A1 also discloses composite materials including at least one nanoparticulate mixed oxide comprising SiO2 and an oxide of any of elements Y, La, Ta, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. However, due to the great difference in the refractive indices of filler materials and matrix, poor transparency has to be expected here likewise.
Instead of the preparation methods for fillers mentioned above, inorganic fillers may also be prepared by chemical precipitation. For example, patent document DE 196 43 781 C2 describes preparation of X-ray opaque spherical particles via a sol-gel route. The particles so obtained include SnO2 and at least one further oxide of the elements of the 1st through 5th main group and/or of the transition metals. However, mandatorily, the particles include SnO2.
Furthermore described in the literature (Wei et al., Journal of Applied Polymer Science, 70, 1689-1699 (1998)) is the preparation of a filler material based on SiO2 using a sol-gel process. In this filler material, polymer chains are covalently bound to the silicon network. In form of a composite with the appropriate matrix monomer, the filler material so obtained exhibits enhanced compressive strength, however, the composite does not include any components to increase radiopacity.